In electrostatic copying apparatus, it is now conventional to use a rotatable magnet roll, coaxial with a surrounding rotatable non-magnetic shell, for transporting magnetic toner from a toner supply to a development area and for further transporting unused toner through a development gap between the shell and an image-bearing drum, on around the shell and magnet roll and back to the toner supply.
It is also known to maintain the toner supply in a tank having a substantial opening in the bottom into which the shell and magnet roll protrude. In such an arrangement, the magnetic toner passes through at least three gaps in which the nonmagnetic shell forms one surface: (1) a gap between the end of an upper wall of the toner tank and the shell for drawing out toner from the tank onto the surface of the non-magnetic shell; (2) a gap between the image-bearing drum and the shell forming a portion of the development area; and (3) a gap between the end of a lower wall of the toner tank and the shell for drawing unused toner back into the toner supply tank. The ends of the upper wall and the lower wall form the opening in the tank into which the shell and magnetic roll partially protrude.
In this type of arrangement however, the magnetic toner tends to leak through the gaps between the walls of the tank and the non-magnetic drum when new toner is being supplied to the tank. The leakage is particularly troublesome as to the gap between the lower wall of the tank and the shell, when the shell and the magnet roll are stationary.